Since fluorine-containing elastomers, including vinylidene fluoride-hexafluoropropylene (VdF-HFP) elastomers and tetrafluoro-ethylene (TFE)-perfluorovinyl ether elastomers, have excellent chemical resistance, solvent resistance and heat resistance, they have widely been used in the fields of automobile, semiconductor, chemical or the like industries as O-rings, gaskets, hoses, stem sealing, shaft sealing, diaphragm, and so on, which are used in severe environment.
Fluorine-containing elastomers used in such applications include iodine and fluorine-containing elastomers having highly active iodine atoms at the molecular ends. The iodine atoms at the molecular ends enable the iodine and fluorine-containing elastomers to have a good crosslinking efficiency, and thus the elastomers have an excellent vulcanizability. Furthermore, since there is no need to add chemical substances containing a metal component, the elastomers have been widely used as peroxide vulcanization molded articles.
A peroxide vulcanization type (see, for example, Japanese Unexamined Patent Publication No. 53-125491) is excellent in chemical resistance and steam resistance (hot water resistance), but is inferior in compression set to a polyol vulcanization type, and therefore it has not been suitable for use as sealing materials. This problem has been solved by introducing vulcanization sites into the elastomer main chain (see, for example, Japanese Unexamined Patent Publication No. 62-12734). However, the vulcanization density increases to result in lowered elongation at break. Therefore, it has been very difficult to have both excellent compression set and excellent elongation at break.
Meanwhile, as a process for preparing fluorine-containing elastomers by high pressure polymerization are known a polymerization method wherein at least one monomer is in a supercritical state (c.f. pamphlet of WO 00/47641) and an emulsion polymerization method wherein the monomer concentration in polymer particles is at least a specific level (c.f. pamphlet of WO 01/34666). Although these patent publications describe that the polymerization can be carried out in the presence of Rf1·Ix as used in the present invention, they do not disclose any working example therefor and any effects disclosed in the present invention.
Iodine and fluorine-containing elastomers have been prepared by an emulsion polymerization such as a so-called iodine transfer polymerization method (see, for example, Japanese Examined Patent Publication No. 63-41928). According to the method, however, it is necessary to hold down the amount of a polymerization initiator, in order to get a high terminal iodine content (see, for example, Masayoshi Tatemoto, p19, 86/6, Symposium on Micromolecules, Control of Polymer Structure in Radical Polymerization, The Society of Polymer Science, Japan, (1986)), and accordingly the productivity cannot be improved. In polymerization systems wherein the amount of a polymerization initiator is not limited, the polymerization rate can easily be increased by increasing the amount of the initiator, but in iodine transfer polymerization systems, the increase of the amount of initiator cannot be expected, because the initiator terminal content exerts a great influence on physical properties of end products.
Various proposals for improving the productivity have been made. For example, a process for improving the productivity by continuously carrying out emulsion polymerization is proposed (see, for example, Japanese Unexamined Patent Publication Nos. 3-33108 and 3-221510). According to this process, good tensile strength and compression set, which are properties of iodine and fluorine-containing elastomers, cannot be obtained.
Further, a polymerization at a high pressure of at least 1.7 MPa (gauge pressure, hereinafter the same) is proposed (see, for example, Japanese Unexamined Patent Publication No. 5-222130), but the publication disclosed that a pressure range of 2.6 to 2.7 MPa is preferable and the pressure used in the Examples is within that range. Also, the polymerization time is more than 15 hours. Further, a microemulsion polymerization process is proposed (see, for example, Japanese Unexamined Patent Publication No. 63-8406). The process, however, requires the use of a fluorine oil or the like to form a microemulsion at the initial stage. Since the fluorine oil and the like remain in the products obtained therefrom as contamination sources, it is necessary to remove the contamination sources by washing.
In order to stabilize a polymerization system or to increase the rate of polymerization, the amount of an emulsifier might be increased. However, the increase of the amount is undesirable from the economic and environmental points and in addition, it is also necessary to remove the emulsifier by washing since the emulsifier itself impairs curing.
In order to solve these problems, it is proposed to carry out iodine transfer polymerization in a two stage emulsion polymerization manner (see, for example, a pamphlet of WO 00/01741). The two stage emulsion polymerization is a process wherein a large number of polymer particles are synthesized using a relatively large amount of an emulsifier in the first stage, and the resulting emulsion is then diluted to lower the concentration of polymer particles and the concentration of the emulsifier, and the polymerization in the second stage is carried out using the diluted emulsion. This process enables to increase the rate of polymerization twice or more without big change of existing facilities for emulsion polymerization with giving a uniform particle size and keeping the original characteristics. However, the productivity is still inferior to that of polymerization processes using no iodine compound. Further, the elastomers obtained by this process have no improved properties as compared to those obtained by a conventional iodine transfer polymerization process, and still have a problem in the sealing properties as mentioned above.
As described above, there has not yet been provided a process which satisfies both the productivity for the preparation of iodine and fluorine-containing elastomers and the properties thereof.